Ab initio density functional computations of conformations and bond dissociation energies for hexahydro-1,3,5-trinitro-1,3,5-triazine

Ab initio computations are described of the conformers and bond dissociation energies of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) with the Becke3-Lee-Yang-Parr (B3LYP) nonlocal density functional and the standard 6-31G* and 6-311G** Gaussian basis sets. Five conformational minima, closely spaced in energy, were located at the B3LYP/6-31G* level of theory. The experimental gas-phase and solid-phase geometries for RDX are compared with the B3LYP/6-31G* geometries. The rather short N-N bonds in the experimental crystal structure suggest that intermolecular interactions lead to enhanced donation of the amine lone pair into the nitro group in the solid state. The RDX nitrogen and carbon radicals have key roles in thermal decomposition of RDX. The B3LYP/6-311G** computed estimates for the N-NO₂ and the C-H dissociation energies (D(o)) in RDX are respectively 42 and 85 kcal/mol. Compared with computed N-NO₂ and C-H bond strengths in related molecules, the bonds in RDX are rather weak. This suggests that initiation of decomposition by N-N cleavage and propagation of the decomposition by hydrogen atom transfers should be facile.